Exceptional Control Flow Part I Today Exceptions Process context - - PowerPoint PPT Presentation

Chris Riesbeck, Fall 2011 Original: Fabian Bustamante

Exceptional Control Flow Part I

Today Exceptions Process context switches Creating and destroying processes Next time Signals, non-local jumps, …

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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Control flow

Computers do only one thing

– From startup to shutdown, a CPU simply reads and executes (interprets) a sequence of instructions, one at a time. – This sequence is the system’s physical control flow (or flow of control).

<startup> inst1 inst2 inst3 … instn <shutdown> Physical control flow Time

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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Altering the control flow

Up to now: two mechanisms for changing control flow

– Jumps and branches – Call and return using the stack discipline. – Both react to changes in program state.

Insufficient for a useful system

– Difficult for the CPU to react to changes in system state.

• Data arrives from a disk or a network adapter.
• Instruction divides by zero
• User hits ctl-c at the keyboard
• System timer expires

System needs mechanisms for “exceptional control flow”

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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Exceptional control flow

Mechanisms for exceptional control flow exists at all levels of a computer system Low level mechanism

– Exceptions

• change in control flow in response to a system event (i.e.,

change in system state)

– Combination of hardware and OS software

Higher level mechanisms

– Process context switch – Signals – Nonlocal jumps (setjmp/longjmp) – Implemented by either:

• OS software (context switch and signals).
• C language runtime library: nonlocal jumps.

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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System context for exceptions

Local/IO Bus Memory Network adapter IDE disk controller Video adapter Display Network Processor Interrupt controller SCSI controller SCSI bus Serial port controller Parallel port controller Keyboard controller Keyboard Mouse Printer Modem disk disk CDROM

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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Exceptions

Exception – a transfer of control to the OS in response to some event (i.e., change in processor state)

User Process OS exception exception processing by exception handler exception return (optional) event

current next

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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Interrupt vectors

Each type of event has a unique exception number k Index into jump table (a.k.a., interrupt vector) Jump table entry k points to a function (exception handler). Handler k is called each time exception k

• ccurs.

interrupt vector

1 2

...

n-1

code for exception handler 0 code for exception handler 1 code for exception handler 2 code for exception handler n-1

...

Exception numbers

Monday, November 21, 2011

Exceptions Exception numbers created by

– processor designers – OS kernel designers

Exception handling

– like procedure call – return address pushed on stack – might be current instruction or next, depending on type of exception – additional processor state pushed, e.g., condition flags – data be pushed on either user stack or kernel stack – handler run in kernel mode

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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Asynchronous exceptions (Interrupts)

Caused by events external to the processor

– Indicated by setting the processor’s interrupt pin – handler returns to “next” instruction.

Examples:

– I/O interrupts

• hitting ctl-c at the keyboard
• arrival of a packet from a network
• arrival of a data sector from a disk

– Hard reset interrupt

• hitting the reset button

– Soft reset interrupt

• hitting ctl-alt-delete on a PC

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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Synchronous exceptions

Caused by events that occur as a result of executing an instruction:

– Traps

• Intentional
• Examples: system calls, breakpoint traps, special instructions
• Like procedure call but in kernel mode
• Returns control to “next” instruction

– Faults

• Unintentional but possibly recoverable
• Examples: page faults (recoverable), protection faults

(unrecoverable).

• Either re-executes faulting (“current”) instruction or aborts.

– Aborts

• unintentional and unrecoverable
• Examples: parity error, machine check.
• Aborts current program

Monday, November 21, 2011

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Trap example

Opening a File

– User calls open(filename, options)

• Function open executes system call instruction int

– OS must find or create file, get it ready for reading or writing – Returns integer file descriptor

User Process OS exception Open file return

int pop

0804d070 <__libc_open>: . . . 804d082: cd 80 int $0x80 804d084: 5b pop %ebx . . .

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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Fault example #1

Memory reference

– User writes to memory location – That portion (page) of user’s memory is currently on disk – Page handler must load page into physical memory – Returns to faulting instruction – Successful on second try

User Process OS page fault Create page and load into memory return event

movl

int a[1000]; main () { a[500] = 13; } 80483b7: c7 05 10 9d 04 08 0d movl $0xd,0x8049d10

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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Fault example #2

Memory reference

– User writes to memory location – Address is not valid – Page handler detects invalid address – Sends SIGSEG signal to user process – User process exits with “segmentation fault”

int a[1000]; main () { a[5000] = 13; } 80483b7: c7 05 60 e3 04 08 0d movl $0xd,0x804e360

User Process OS page fault Detect invalid address event

movl

Signal process

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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Processes

Def: A process is an instance of a running program.

– One of the most profound ideas in computer science. – Not the same as “program” or “processor”

Process provides each program with two key abstractions:

– Logical control flow

• Each program seems to have exclusive use of the CPU.

– Private address space

• Each program seems to have exclusive use of main memory.

How are these illusions maintained?

– Process executions interleaved (multitasking) – Address spaces managed by virtual memory system

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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Logical control flows

Time Process A Process B Process C

Each process has its own logical control flow

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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Concurrent processes

Two processes run concurrently (are concurrent) if their flows overlap in time. Otherwise, they are sequential. Examples:

– Concurrent: A & B, A & C – Sequential: B & C

Time Process A Process B Process C

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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User view of concurrent processes

Control flows for concurrent processes are physically disjoint in time. However, we can think of concurrent processes are running in parallel with each other.

Time Process A Process B Process C

Monday, November 21, 2011

Checkpoint

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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Context switching

Processes are managed by a shared chunk of OS code called the kernel

– Not a separate process, but runs as part of user process

Control flow passes from one process to another via a context switch. A context is all the data needed to restart a process, e.g., register values, stack values, page table, …

Process A code Process B code

user code kernel code user code kernel code user code

Time

context switch context switch

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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Private address spaces

Each process has its own private address space.

kernel virtual memory (code, data, heap, stack) memory mapped region for shared libraries run-time heap (managed by malloc) user stack (created at runtime) unused %esp (stack pointer) memory invisible to user code brk 0xc0000000 0x08048000 0x40000000 read/write segment (.data, .bss) read-only segment (.init, .text, .rodata) loaded from the executable file 0xffffffff

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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fork: Creating new processes

int fork(void)

– creates a new process (child process) that is identical to the calling process (parent process) – returns 0 to the child process – returns child’s pid to the parent process

if (fork() == 0) { printf("hello from child\n"); } else { printf("hello from parent\n"); } Fork is interesting (and often confusing) because it is called

• nce but returns twice

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EECS 213 Introduction to Computer Systems Northwestern University

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Fork example #1

Key points

– Parent and child both run same code

• Distinguish parent from child by return value from fork

– Start with same state, but each has private copy

• Including shared output file descriptor
• Relative ordering of their print statements undefined

void fork1() { int x = 1; pid_t pid = fork(); if (pid == 0) { printf("Child has x = %d\n", ++x); } else { printf("Parent has x = %d\n", --x); } printf("Bye from process %d with x = %d\n", getpid(), x); }

Book uses Fork() wrapper function. See 8.3 for why it's important.

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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Fork example #2

Key points

– Both parent and child can continue forking

void fork2() { printf("L0\n"); fork(); printf("L1\n"); fork(); printf("Bye\n"); }

L0 L1 L1 Bye Bye Bye Bye

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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Fork example #3

Key points

– Both parent and child can continue forking

void fork3() { printf("L0\n"); fork(); printf("L1\n"); fork(); printf("L2\n"); fork(); printf("Bye\n"); }

L1 L2 L2 Bye Bye Bye Bye L1 L2 L2 Bye Bye Bye Bye L0

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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Fork example #4

Key points

– Both parent and child can continue forking

void fork4() { printf("L0\n"); if (fork() != 0) { printf("L1\n"); if (fork() != 0) { printf("L2\n"); fork(); } } printf("Bye\n"); }

L0 L1 Bye L2 Bye Bye Bye

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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Fork example #5

Key points

– Both parent and child can continue forking

void fork5() { printf("L0\n"); if (fork() == 0) { printf("L1\n"); if (fork() == 0) { printf("L2\n"); fork(); } } printf("Bye\n"); }

L0 Bye L1 Bye Bye Bye L2

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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exit: Destroying process

void exit(int status)

– exits a process

• Normally return with status 0

– atexit() registers functions to be executed upon exit

void cleanup(void) { printf("cleaning up\n"); } void fork6() { atexit(cleanup); fork(); exit(0); }

Monday, November 21, 2011

Checkpoint

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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Zombies

Idea

– When process terminates, still consumes system resources

• Various tables maintained by OS

– Called a “zombie”

• Living corpse, half alive and half dead

Reaping

– Performed by parent on terminated child, using wait – Parent is given exit status information – Kernel discards process

What if parent doesn’t reap?

– If any parent terminates without reaping a child, then child will be reaped by the kernel's init process (PID = 1) – Only need explicit reaping for long-running processes

• E.g., shells and servers

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

28 linux> ./forks 7 & [1] 6639 Running Parent, PID = 6639 Terminating Child, PID = 6640 linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6639 ttyp9 00:00:03 forks 6640 ttyp9 00:00:00 forks <defunct> 6641 ttyp9 00:00:00 ps linux> kill 6639 [1] Terminated linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6642 ttyp9 00:00:00 ps

Zombie - Example

ps shows child process as “defunct” Killing parent allows child to be reaped

void fork7() { if (fork() == 0) { /* Child */ printf("Terminating Child, PID = %d\n", getpid()); exit(0); } else { printf("Running Parent, PID = %d\n", getpid()); while (1) ; /* Infinite loop */ } } Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

29 linux> ./forks 8 Terminating Parent, PID = 6675 Running Child, PID = 6676 linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6676 ttyp9 00:00:06 forks 6677 ttyp9 00:00:00 ps linux> kill 6676 linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6678 ttyp9 00:00:00 ps

Nonterminating child example

Child process still active even though parent has terminated Must kill explicitly, or else will keep running indefinitely

void fork8() { if (fork() == 0) { /* Child */ printf("Running Child, PID = %d\n", getpid()); while (1) ; /* Infinite loop */ } else { printf("Terminating Parent, PID = %d\n", getpid()); exit(0); } } Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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wait: Synchronizing with children

pid_t wait(int *child_status)

– suspends current process until one of its children terminates – return value is the pid of the child process that terminated – can happen in any order – if child_status != NULL, then the object it points to will be set to a status indicating why the child process terminated

pid_t waitpid(pid_t pid, &status, options)

– wait for specific process, various options – wait(&status) ≡ waitpid(-1, &status, 0) Use macros WIFEXITED and WEXITSTATUS from <sys/wait.h> to interpret exit status Both return -1 if error, e.g., – wait() error if process has no child – waitpid() error if pid is not a child of this process

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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wait: Synchronizing with children

void fork9() { int child_status; if (fork() == 0) { printf("HC: hello from child\n"); } else { printf("HP: hello from parent\n"); wait(&child_status); printf("CT: child has terminated\n"); } printf("Bye\n"); exit(); }

HP HC Bye CT Bye

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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wait Example

void fork10() { pid_t pid[N]; int i; int child_status; for (i = 0; i < N; i++) if ((pid[i] = fork()) == 0) exit(100+i); /* Child */ for (i = 0; i < N; i++) { /* whichever ends first */ pid_t wpid = wait(&child_status); if (WIFEXITED(child_status)) printf("Child %d terminated with exit status %d\n", wpid, WEXITSTATUS(child_status)); else printf("Child %d terminate abnormally\n", wpid); } }

Child 3565 terminated with exit status 103 Child 3564 terminated with exit status 102 Child 3563 terminated with exit status 101 Child 3562 terminated with exit status 100 Child 3566 terminated with exit status 104

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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waitpid

void fork11() { pid_t pid[N]; int i; int child_status; for (i = 0; i < N; i++) if ((pid[i] = fork()) == 0) exit(100+i); /* Child */ for (i = 0; i < N; i++) { pid_t wpid = waitpid(pid[i], &child_status, 0); if (WIFEXITED(child_status)) printf("Child %d terminated with exit status %d\n", wpid, WEXITSTATUS(child_status)); else printf("Child %d terminated abnormally\n", wpid); }

Child 3568 terminated with exit status 100 Child 3569 terminated with exit status 101 Child 3570 terminated with exit status 102 Child 3571 terminated with exit status 103 Child 3572 terminated with exit status 104

Monday, November 21, 2011

Checkpoint

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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exec: Running new programs

int execl(char *path, char *arg0, char *arg1, …, 0) – loads and runs executable at path with args arg0, arg1, …

• path is the complete path of an executable
• arg0 becomes the name of the process

– typically arg0 is either identical to path, or else it contains only the executable filename from path

• “real” arguments to the executable start with arg1, etc.
• list of args is terminated by a (char *)0 argument

– returns -1 if error, otherwise doesn’t return!

One of a family of exec function front-ends to execve()

main() { if (fork() == 0) { execl("/usr/bin/cp", "cp", "foo", "bar", 0); } wait(NULL); printf("copy completed\n"); exit(); }

Monday, November 21, 2011

EECS 213 Introduction to Computer Systems Northwestern University

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Summarizing

Exceptions

– Events that require nonstandard control flow – Generated externally (interrupts) or internally (traps and faults)

Processes

– At any given time, system has multiple active processes – Only one can execute at a time, though – Each process appears to have total control of processor + private memory space

Spawning processes

– Call to fork: one call, two returns

Terminating processes

– Call exit: one call, no return

Reaping processes

– Call wait or waitpid

Replacing program executed by process

– Call execl (or variant): one call, (normally) no return

Monday, November 21, 2011